Recently, there has been an explosive increase in the demand for wireless data services. Further, an evolution from a wireless voice service to a wireless data service requires a gradual increase of wireless capacity. To cope with such a demand, wireless service providers and wireless equipment manufacturers attempt to improve a data rate of a wireless system, which results in motivating massive researches. A wireless channel experiences various problems such as path loss, shadowing, fading, noise, limited bandwidth, power limit of a user equipment (UE), and interference between other users. Due to these limitations, the wireless channel has a shape of a narrow pipe that obstructs fast data flow, and it is difficult to design an effective bandwidth of wireless communication providing high-speed data transmission. The designing of the wireless system has other challenges such as resource allocation, mobile issues in association with a rapidly changing physical channel, portability, security, and privacy.
When a transport channel experiences deep fading, if a different version or a replica of a transmitted signal is not additionally transmitted, it is difficult for a receiver to determine the transmitted signal. A resource corresponding to the different version or the replica is referred to as diversity, and is one of most important factors that contribute to reliable transmission over a wireless channel. The use of the diversity can maximize data transfer capacity or data transfer reliability. A system for implementing the diversity by using multiple transmit (Tx) antennas and multiple receive (Rx) antennas is referred to as a multiple input multiple output (MIMO) system. The MIMO system is also referred to as a multiple antenna system.
A multiple antenna scheme includes space frequency block coding (SFBC), space time block coding (STBC), cyclic delay diversity (CDD), frequency switched transmit diversity (FSTD), time switched transmit diversity (TSTD), precoding vector switching (PVS), spatial multiplexing (SM), generalized cyclic delay diversity (GCDD), selective virtual antenna permutation (S-VAP), etc.
Such a MIMO scheme is also considered as a measure for improving a data rate and reliability in a communication system having a multiple cell structure. Meanwhile, researches on a cooperative transmission method are actively conducted for transmission of a signal through mutual cooperation between cells or between base stations (BSs) in the communication system having the multiple cell structure. The UE can improve not only a Tx power gain but also signal sensitivity by communicating with a plurality of BSs.
In general, before performing data transmission/reception with the BS, the UE measures channel quality of each cell by using a reference signal transmitted from the BS. Further, the UE configures a radio connection with a cell determined by the measurement. The radio connection may also be referred to as a radio link or a radio resource control (RRC) connection. The UE may persistently perform the measurement even after the radio connection is configured. This is because the measurement provides the BS with a variety of information required for proper network management and resource allocation in consideration of a radio environment.
However, in the cooperative transmission method, the UE configures and/or reconfigures a radio connection with a plurality of cells instead of one cell. In particular, it is preferable to reliably maintain a radio connection between the UE and the BS in order to continuously transmit and receive data in the process of reconfiguring the radio connection. Therefore, there is a need for a method capable of configuring or reconfiguring a radio connection with a plurality of cells without data loss in the cooperative transmission method.